EVOLUTION OF THE USE OF MINERALOGIC INFORMATION: BIOAPATITE IN TOOTH ENAMEL

Decades ago mineralogist Duncan McConnell first proposed the existence of carbonated apatite in the geologic environment. Later research revealed that bioapatite is also carbonated. Remarkably, the crystal-chemistry of nanocrystalline bioapatite is still not clear. Although the details of how that chemistry is accommodated crystallographically in the mineral component of bone, dentin, and enamel remain elusive, we nevertheless have come to take geochemical advantage of the variability of bioapatite’s crystal-chemistry. For instance, trace-element and isotopic analyses of fossil tooth enamel are used as proxies to reveal the nature of the environment in which the original animal lived. Moreover, renewed interest in the mineralogy and chemistry of enamel has been spurred by the desire to obtain even more accurate environmental information and to use spatially resolved analyses on enamel in order to derive temporally (i.e., growth) resolved information. We have applied Raman spectroscopic analysis to numerous types of bioapatite, thereby establishing spectroscopic parameters whose range of values define and differentiate the apatite in bone, dentin, and enamel. Knowledge of the range of values for modern enamel permits comparison with fossil enamel. Our initial goal was to test if Raman microprobe spectroscopy could be used for rapid, non-destructive assessment of whether fossil enamel had undergone geochemical changes since the animal died. A pilot study provided good evidence that Raman spectroscopy can indicate on a micrometer scale which portion of the enamel in a fossil tooth is most pristine and therefore best suited for further geochemical analyses. Our more recent studies show considerable spectroscopic differences (especially in the carbonate and hydroxyl concentrations) in the enamel of different mammals, e.g., horse, bison, beaver, water buffalo, and human. Mineral-chemical differences in the bioapatite suggest that the enamel of some animals may be geochemically more robust than of others once the teeth enter the soil environment. Experimental exchange reactions with enamel from different animals could shed light on which animals’ fossil teeth are geochemically most informative, i.e., least readily altered by diagenesis.